This invention relates in general to rotor systems for helicopters and deals more particularly with improved hub assemblies for such rotor systems.
Rotor hub structures for large helicopters are generally quite heavy and costly to manufacture. A CH-47C helicopter, for example, may be provided with a hub machined from a 4340-steel forging which weighs in excess of 700 pounds and requires more than 70 machining operations to finish. Further, the size of conventional hubs for large helicopters approach the limits of forging feasibility.
The trend in metal rotor-hub material has been toward titanium alloys, which provide high strength-to-weight ratios and good corrosion resistance. Although lighter than their steel counterparts, titanium alloy hubs are more costly. A typical hub structure of the type with which this invention is concerned is used on a CH-54B helicopter, a heavy-lift helicopter which has a single main rotor with six articulated rotor blades. Its titanium-alloy main rotor hub is five feet wide and one foot high. The existing production hub has for this helicopter two structural elements, an upper hub and a lower plate. The upper hub consists of six cantilevered beams that radiate from a hollow circular cylinder. The lower plate is similarly star-shaped but much thinner. The beams of the upper hub alone support shears from lift and control movements. The hub is connected to the drive shaft by splines for torque transfer, cone seats for moment and a threaded nut for axial force. This configuration is effective and provides compact load paths that are appropriate for a dense material that exhibits high strength, and specific stiffness. However, this hub contributes substantially to the weight of the aircraft, utilizes costly materials, and requires extensive machining operations to manufacture.
It is the general aim of the present invention to provide improved hub assemblies for rotor systems which weigh less than comparable existing hub structures, cost less to manufacture, and which possess the requisite strength and durability of present comparable production hubs. A further aim of the invention is to provide improved hub structures which provide a high degree of damage tolerance, which are simple to maintain, and which may be readily installed and removed without special tools.